On Feb. 21, 2003, a 65-year-old physician who lived in the Chinese province that abuts Hong Kong crossed into the territory surrounding the city and checked into a hotel in Kowloon. He was given a room on the ninth floor. Sometime during his stay — no one has ever fully traced his path — he encountered roughly a dozen other people; most of them were hotel guests whose rooms were on the same floor, but some were staying on other floors, and some were visitors to events there. The physician had been sick for a week with symptoms that had started like the flu, but were turning into pneumonia, and the next day, he checked out of the hotel and went to a Hong Kong hospital. Before the end of the day, he died.
In the next few days, the people who had crossed paths with the physician left the hotel. Most of them were visitors to the special administrative region: Hong Kong is not only a port and transit hub, but a business and shopping destination for much of the Pacific Rim. They went to Vietnam, Singapore, Canada, and Ireland. As they traveled, some of them started to feel as though they had picked up the flu.